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67 5.3 EARTHQUAKES Earthquakes occur when two tectonic plates slip past each other beneath the earth’s surface, causing sudden and rapid shaking of the surrounding ground. Earthquakes originate on fault planes below the surface, where two or more plates meet. As the plates move past each other, they tend to not slide smoothly and become “locked,” building up stress and strain along the fault. Eventually the stress causes a sudden release of the plates, and the stored energy is released as seismic waves, causing ground acceleration to radiate from the point of release, the “epicenter.” The Bay Area is in the heart of earthquake country. Major faults cross through all nine Bay Area counties. Every point within the Bay Area is within 30 miles of an active fault, and 97 of the 101 cities in the Bay Area are within ten miles of an active fault. Figure 5-2 shows the location of active faults that are mapped near San Mateo under the Alquist-Priolo Act. The Hayward Fault runs directly through San Leandro, and has the potential for significant damage to the city if a major earthquake were to occur (as expected) along the Hayward Fault. The San Andreas Fault, located approximately 14 miles west of the San Leandro, depending upon magnitude, could produce similar damage to the City as the 1989 Loma Prieta earthquake did. The total amount of energy released in an earthquake is described by the earthquake magnitude. The moment magnitude scale (abbreviated as M) is logarithmic; the energy released by an earthquake increases logarithmically with each step of magnitude. For example, a M6.0 earthquake releases 33 times more energy than a M5.0, and a M7.0 earthquake releases 1,000 times more energy than a M5.0 event. The quantified size or measurement of an earthquake is dependent on factors that include the length of the fault and the ease with which the plates slip past one another. In the Bay Area, technical specialists have observed varied fault behaviors, giving some sense of which faults may or may not produce a large, damaging earthquake. Earth scientists are most concerned about the San Andreas and Hayward faults, believed most likely to produce large, regionally damaging earthquakes. There are, however, many other Bay Area faults that can produce localized damage. Additionally, earthquakes are often not isolated events, but are likely to trigger a series of smaller aftershocks along the fault plane, which can continue for months to years after a major earthquake, producing additional damage. The energy released in earthquakes can produce five different types of hazards:  Surface Fault rupture  Ground shaking  Liquefaction  Earthquake-induced landslides  Tsunamis and seiches 106